This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-099596, filed Mar. 30, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a power factor control apparatus and method for controlling power factors of a plurality of power generators operated in parallel.
2. Description of the Related Art
Conventionally, when a plurality of power generators, which are not connected to a system of an electric power company, are operated in parallel (system-unconnected parallel operation), the balance between the power factors of the power generators is maintained by a PFCC (power factor compensating circuit) as will be described below.
FIG. 4 is a diagram showing the structure of a reactive current control apparatus, to which a PFCC disclosed in Jpn. Pat. Appln. KOKAI Publication No. 51-103209 is applied. In this apparatus, when there is a difference between the power factors of two power generators 11a and 11b, the outputs of power factor detectors 12a and 12b corresponding to the power generators 11a and 11b are different. At this time, the power factor of a load (bus) 10a has the mean value of the power factors of the power generators 11a and 11b. Therefore, the output from the power factor detector 12c corresponding to the bus 10a has the mean value of the outputs of the power factor detectors 12a and 12b, and a differential signal between the power factor detectors 12c and 12a has a polarity opposite to that of a differential signal between the power factor detectors 12c and 12b. 
In this state, when voltages of the power generators 11a and 11b are adjusted, signals of automatic voltage regulators 13a and 13b have opposite polarities. In other words, the voltage of one of the power generators 11a and 11b is raised, while the voltage of the other is lowered, thereby equalizing the power factors of the power generators 11a and 11b. As a result, the power factor balance between the power generators can be maintained without control from external equipment.
However, in the conventional control method described above, if a plurality of light-duty power generators have to cope with the work in a factory, for example, the following problems will arise: since a plurality of signals are successively output to the power generators from automatic voltage regulators, the power generators cannot cope with sudden load fluctuation.
Further, when the power generators are connected to a system of an electric power company, if the power factors of the generators do not balance, the power factor balance cannot be recovered if the power generators are disconnected from the system due to a power failure or the like and operated again in isolation. In this case, over-excitation or an overload condition may occur, resulting in a trip.
The control method described above is effective when the power generators have the same specification and the same capacity. However, if the power generators have different specifications (for example, if they are produced by different manufacturers) or different capacities, the following problem may arise. Since the properties of the generators vary due to differences in PFCCs, themselves varying with manufacturer, reactions of the power generators in response to signals from the automatic voltage regulators are not uniform. Therefore, it is not expected that the power factors and the voltages can be kept in balance.
An object of the present invention is to provide a power factor control apparatus and method, which can control power factors of power generators of different specifications or capacities in a system-unconnected parallel operation, while power factors and voltages are kept in balance.
According to an aspect of the present invention, there is provided a power factor control apparatus, which controls power factors of a plurality of power generators connected to a bus, comprising: a target power factor setting section which sets a target power factor range of the plurality of power generators in a fixed cycle; a comparing section which compares a voltage of the bus with a predetermined voltage value range in the fixed cycle; a first control section which performs control, based on a result of the comparing section, to drop a voltage of at least one power generator whose power factor is the lowest, when the voltage of the bus is higher than the predetermined voltage value range, and raise a voltage of at least one power generator whose power factor is the highest, when the voltage of the bus is lower than the predetermined voltage value range; and a second control section which performs control of a voltage of a power generator, whose power factor is out of the target power factor range, such that the power factor thereof falls within the target power factor range, when the voltage of the bus falls within the predetermined voltage value range as a result of the comparing section.